The long-term goal of this research is to understand how testosterone (T) acts on the brain to modify male social behaviors. During the period of requested support, the focus will be on the activation of mating in adult males to capitalize on the identification of cell groups in the gerbil medial preoptic area, and related pathways, that are essential for this behavior. The critical cell groups are the two major parts of the sexually dimorphic area (SDA). The pathways are a projection from the SDA to the retrorubral field (RRF) and a reciprocal connection of the SDA with the medial bed nucleus of the stria terminalis (BSTm). To learn more about the circuitry for male sex behavior, four aims of this research are 1) to test whether SDA efferents to the RRF use gamma-amino-butyric acid (GABA) as their transmitter, 2) to test whether the pedunculopontine tegmental nucleus contributes to any effects attributed to the RRF, 3) to determine if the RRF and caudal BSTm affect mating as parts of a common pathway and 4) to test the hypothesis that the medial part of the medial preoptic nucleus of rats is analogous to the gerbil medial SDA. Another cell group, the SDA parts compacta (SDApc), which is seen almost exclusively in males in adulthood, is not essential for male sex behavior. Aim 5 is to test whether it affects other aspects of reproduction. SDA cell groups also show specific patterns of activation during mating as assessed by c-Fos immunocytochemistry. The medial SDA responds when males enter the sexual context and when they ejaculate. A thalamic nucleus connected to the mSDA shows the same pattern. Part of the posterodorsal medial amygdala (MeApd), which is connected to the mSDA, and the posterodorsal preoptic nucleus, which is near the SDA, respond only with ejaculation. The remaining aims of the research are 6) to test whether either the PdPN or MeApd is causally related to ejaculation (behavior or seminal emission), 7) to assess the relationships among the four cell groups activated with ejaculation and 8) to determine if the MeApd cells involved contain any of the peptides that are dimorphic or T sensitive in the MeApd of other species. The expression of sex behavior can clearly affect both the mental and physical health of men and their partners.